The present invention relates to a vehicle steering apparatus.
A steering control system, which controls the yaw moment of a vehicle by controlling the steering angle of steered wheels based on a vehicle model (vehicle motion model), has been proposed in recent years (refer to, for example, Japanese Laid-Open Patent Publication No. 2002-254964). The vehicle model is generated by modeling the relationship between vehicle-condition parameters, such as vehicle velocity and yaw rate, and motion states of the vehicle. An intelligent front steering (IFS) system is one example of such a steering control system.
The IFS system integrally executes electronic control associated with steered wheels, such as variable gear ratio control and power assist control, based on a plurality of vehicle-condition parameters. The variable gear ratio control changes the transmission ratio (gear ratio) of the steered wheels relative to the turning angle of the steering wheel. The power assist control applies an assist force to the steering system of the vehicle.
For example, the IFS system calculates target values (e.g., a target yaw rate) of vehicle-condition parameters based on the vehicle model. Based on the target values, the system determines the steering characteristic of the vehicle and calculates a control amount for the steered wheels. When the vehicle is in an understeer state, the system executes such control that reduces the turning angle of each steered wheel (understeer control). When the vehicle is in an oversteer state, the system executes such control that steers each steered wheel at an angle oriented in a direction opposite to the direction of the actual steering angle. That is, the system executes control for counter-steering (oversteer control). This stabilizes the behavior of the vehicle even under conditions where the behavior of the vehicle is likely to become unstable, such as on a low-friction road.
Such understeer control and oversteer control are executed by changing an ACT angle, which is a steering angle for the steered wheels produced by motor drive, by actuating a variable gear ratio actuator. Thus, the steering operation performed by the driver does not necessarily have a constant relationship with the movement of the steered wheels. The expected steering torque may differ from the actual steering torque. This may result in an uncomfortable steering feel.